New stage

Two years ago, I was a college students student. Up until now I've been a primary school teacher for almost two years. It's really very remarkble for me that I got the chance to be a teacher. I like my boys and girls, which is who are lovely, naive and really creative, although sometimes they are naughty and brainless. (Seems rude, as if you're calling them stupid)  When I teach them, I always learn some new knowledge ( I would say 'learn new things', knowledge sounds unnatural) at the same time. They teach me, too. 

This time, I come back with my new goal:be the best teacher I can be. <----- I'm not sure what you mean,

Are you going to a new school, or have you just came back from holiday? 

When I get back, I have a goal; be the best teacher I can be.

Or are you making a new goal?

Now, I have a new goal; be the best teacher I can be.

I noticed that your tenses are a little mixed up, in English we tend to use one tense for one sentence and try to stick to that tense for a whole piece of text. 

Also, I'm being super picky, but you need spaces after punctuation. So  . , ! ? ; : 

But it's still really good. :D

New stage

Two years ago, I was a college students. Now, I've been a primary school teacher for almost two years .It's really very remarkable to me that I got the chance to be a teacher.I like my boys and girls, which is who are lovely, naive(innocent is a better word here) and really creative. Although, sometimes they are naughty and brainless(silly). When I teach them, I always gain some new knowledge at the same time. They teach me, too. This time, I come back with my new goal: to be the best teacher I can be.

Good work! :)